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The Food and Drug Administration took a "big tent" approach earlier this month when it approved two new forms of birth control that prevent pregnancy in very different ways.

Women's health advocates applauded the availability of a new vaginal ring that could be used for up to a year. But some questioned the approval of a mobile phone app that helps women avoid pregnancy by tracking their body temperature and menstrual cycle, a type of contraception called "fertility awareness."

Critics pointed to reports that three dozen women in Sweden got pregnant despite monitoring their cycle with the app. They also fear that the FDA approval of the app may encourage patients to think that fertility awareness methods, which include a range of practices to track ovulation, and avoid unprotected sex during that time, are just as good at preventing pregnancy as some highly effective types of birth control, like the intrauterine device, or IUD. While "natural" methods can be successful, they generally require close daily attention.

There's still room for improvement in contraceptive use by women and men. Nearly half of the 6.1 million pregnancies in the United States — 45 percent — in 2011 were unplanned, according to a study published in the New England Journal of Medicine. That figure is lower than the 51 percent rate in 2008, but is higher than the rate in many other industrialized countries.

The FDA has approved nearly two dozen contraceptive methods, including the pill, the patch, IUDs and hormonal implants and shots, among others. Insurance is required to cover all FDA-approved methods without charging women anything out-of-pocket.

The new vaginal ring, Annovera, releases hormones that prevent ovulation and must be removed after three weeks for seven days, then reinserted. It can be used for a year. The device will not be on the market until at least late 2019, and the price hasn't been released by the manufacturer.

The Natural Cycles app instructs women to take their temperature at the same time every morning when they awake and record it in the app. They also track information about their menstrual cycle. Based on slight temperature changes around ovulation, the app signals when women should avoid unprotected sex. It costs about $80 a year.

Both of the new methods require more attention on the part of the user than say, an IUD, which once inserted can be ignored and is designed to prevent pregnancy for five to 10 years, depending on the brand.

Still, some women's health experts worry that the FDA stamp of approval may be misinterpreted by some women.

"People will interpret this to mean that the FDA approves this and thinks it's a good method," said Dr. Christopher Zahn, vice president of practice activities for the American College of Obstetricians and Gynecologists.

"That's why counseling is so important," he said, noting that doctors should discuss all forms of birth control with women, and the conversation should include the efficacy of different methods.

But Dr. Gillian Dean, senior director of medical services at Planned Parenthood Federation of America, welcomes the approval of both new methods.

"More options are always better," she said. "It isn't one size fits all, and more options increases the likelihood that women will find a method that works for their needs."

The right contraceptive depends on a woman's goals, Dean said, including her reproductive plans, what her menstrual cycle is like, the number of partners she has and how important it is for her not to get pregnant. She said most women who visit Planned Parenthood clinics ask for and receive birth control pills, but an increasing number are asking for long-acting reversible methods of contraception, such as IUDs and hormonal implants.

The IUD and hormonal implants have a "failure rate" of less than 1 percent, making them among the most effective ways of preventing pregnancy (on par with permanent sterilization). Birth control pills, the patch and the vaginal ring have effectiveness rates of about 91 percent, according to the federal Centers for Disease Control and Prevention.

Fertility awareness methods, on the other hand, have a failure rate of about 24 percent, according to the CDC. But that figure is widely misunderstood, said Chelsea Polis, a senior research scientist at the Guttmacher Institute, a sexual and reproductive health research and advocacy organization.

Polis co-authored an analysis of studies of fertility awareness-based contraceptive methods that was published in August in the journal Obstetrics and Gynecology.

The 24 percent figure, she said, primarily reflects the expected failure rate for women who used the rhythm method, a calendar-based approach to calculating when ovulation occurs, rather than newer biometric methods that track body temperature, cervical mucous or urinary hormones. Some of those methods may be more effective, she said.

Based on a review of published studies, Polis and colleagues reported that the Natural Cycles app had a 9.8 percent unintended pregnancy rate. The FDA announcement, which includes the results of an additional study, noted a 6.5 percent rate.

Polis said her research indicates that about 3 percent of women who use contraception practice fertility awareness-based methods, either alone or with other types of birth control, and their numbers are growing.

"I think [the app approval] is largely a positive step forward," Polis said. "I'm relieved that the FDA has a regulatory pathway to evaluate these uses and claims."